Does evolution occur in rapid bursts or gradually? This question is difficult
to answer because we can't replay the past with a stopwatch in hand.
However, we can try to figure out what patterns we'd expect to observe
in the fossil record if evolution did happen in bursts, or if evolution
happened gradually. Then we can check these predictions against what we
observe.

What should we observe in the fossil record if evolution is slow and steady?
If evolution is slow and steady, we'd expect to see the entire transition,
from ancestor to descendant, displayed as transitional
forms over a long period of time in the fossil record.

In the above example, the preservation of many transitional forms, through
layers representing a length of time, gives a complete record of slow and
steady
evolution.

In fact, we see many examples of transitional
forms in the fossil record. For example, to the right we show
just a few steps in the evolution of whales from land-dwelling
mammals, highlighting the transition of the walking
forelimb to the flipper.

Transitional forms in whale evolution

What would we observe in the fossil record if evolution happens in "quick" jumps
(perhaps fewer than 100,000 years for significant change)?
If evolution happens in "quick" jumps, we'd expect to see big changes happen quickly in the fossil record, with little transition between ancestor and descendant.

In the above example, we see the descendant preserved in a layer directly after the ancestor, showing a big change in a short time, with no transitional forms.

When evolution is rapid, transitional forms may not be preserved, even if
fossils are laid down at regular intervals. We see many examples of
this "quick" jumps pattern in the fossil record.

Does a jump in the fossil record necessarily mean that evolution has happened in a "quick" jump?
We expect to see a jump in the fossil record if evolution has occurred as
a "quick" jump, but a jump in the fossil record can also
be explained by irregular fossil preservation.

This possibility can make it difficult to conclude that evolution has happened
rapidly.

We observe examples of both slow, steady change and rapid, periodic change
in the fossil record. Both happen. But scientists are trying to determine
which pace is more typical of evolution and how each sort of evolutionary
change happens.